1. Field of the Invention
The invention relates to light-emitting devices, and in particular to light-emitting devices utilizing gaseous sulfur compounds, wherein a discharge chamber is provided with no plasma-media contacting electrodes built inside the chamber.
2. Description of the Related Art
There are various types of lighting sources, e.g., an incandescent lamp using radiation associated with a burning filament, a fluorescent lamp composed of an electric discharge tube and a fluorescent-powder coating for energy conversion, a high-intensity-discharge (HID) lamp that induces electrical discharge within a highly-pressurized gas or steam, and an electrodeless plasma lighting system (PLS) lamp that generates lighting plasma of gaseous media with no media-contacting electrodes.
The various types of lamps have their respective advantages. For example, incandescent lamps are excellent in color rendition and small in size. Switching circuits of the incandescent lamps are simple and low cost. However, compared to other lamps, incandescent lamps are less power efficient and have a shorter life span. In the other end, fluorescent lamps are more power efficient in emitting light and more durable than other lamps. However, while compared with incandescent lamps, fluorescent lamps are relatively large in size. Additionally, fluorescent lamps require also additional power-ballasting circuits to stabilize discharge current and light output thereof. Other gas-discharge lamps like HID lamps are also power efficient and durable. The HID lamps require, however, a relatively long time for restriking on upon switching off. In addition, HID lamps, similar to fluorescent lamps, requires additional power-ballasting circuits to assist switching. Electrodeless PLS lamps possess longest life among all the above-noted lamps. The electrodeless PLS lamps though are acceptably efficient in emitting light but relatively much expensive. The electrodeless PLS lamps require also additional power-ballasting (though similar but more complex) circuits for switching.
One type of electrodeless PLS lamps, called electrodeless sulfur lamp, is particularly efficient in emitting white light of broadband spectrum even closely resembling to natural sun light.
U.S. Pat. Nos. 5,404,076, 5,594,303, 5,847,517 and 5,757,130, issued to Fusion System Corporation, discloses an electrodeless sulfur lamp.
The electrodeless sulfur lamps disclosed in the above noted US patents consist of a of golf-ball sized quartz bulb containing ten to hundred milligrams of sulfur powers and argon gas at an end of a spindle for rotation. The bulb absorbs microwave energy of 2.45 GHz generated from a magnetron to excite buffering gas of low pressure argon therein and generates gaseous discharging plasma. As a consequent, the space within the quartz bulb is thus supplied with an appropriate amount of free electrons. The sulfur powers absorb the microwave energy to heat and vaporize itself, thereby raising the pressure inside the quartz bulb to 5˜10 times that of the surrounding atmosphere. The gaseous sulfur vapors elevate to a temperature in the quartz bulb under the continuous reaction with microwaves and plasmas of inert buffering gas and are thus stimulated to ionize and discharge. The sulfur ions vigorously oscillate within the space of a narrow mean free path and collapse within itself, thereby causing a molecular-type charge/discharge process, such a process is further aggravated by excitation and collision with highly energetic gas ions in the buffering gas plasma, thereby forming additional luminous thermal plasma of new media and emitting great amounts of photons, having a spectrum of about 73% of visible light, resembling to that of sunlight.
Nevertheless, the electrodeless sulfur lamps disclosed in the above noted US patents need a power source of more than 1.5 KW to reach a luminous efficiency of about 100 lumens per watt. As a result its application is confined to illuminate only large public spaces. In addition, the electrodeless sulfur lamps disclosed by the above noted US patents are normally large in size and appropriate means of electromagnetic shielding in most cases are mandatory, particularly for indoor applications. Therefore, the electrodeless sulfur lamps disclosed by the above noted US patents are not suitable for low power or planar luminance applications.